Electret articles—that is, dielectric articles that exhibit at least quasi-permanent electric charge—are known to exhibit good filtration properties. The articles have been fashioned in a variety of constructions, but for air filtration purposes, the articles commonly take the form of a nonwoven polymeric fibrous web. An example of such a product is the Filtrete™ brand furnace filter sold by the 3M Company. Nonwoven polymeric electret filters are also used in personal respiratory protection devices—see, for example, U.S. Pat. No. 5,307,796 to Kronzer et al., U.S. Pat. No. 5,804,295 to Braun et al., and U.S. Pat. No. 6,216,693 to Rekow et al.
A variety of methods have been used to make electrets, including fiber/electric particle bombardment (U.S. Pat. No. 4,215,682 to Kubik et al.), direct current “DC” corona charging (see, U.S. Pat. No. Re. 30,782 and 32,171 to van Turnhout and U.S. Pat. No. 4,592,815 to Nakao), hydrocharging (see, U.S. Pat. Nos. 5,496,507, 6,119,691, 6,375,886, and 6,783,574 to Angadjivand et al., U.S. Pat. No. 6,406,657 to Eitzman et al., and U.S. Pat. No. 6,743,464 to Insley et al.), and from exposure to polar liquids (U.S. Pat. No. 6,454,986 to Eitzman et al.). The electric charge that is imparted to the dielectric article is effective in enhancing particle capture.
In addition to fibrous structures, microstructured or microchanneled filter media have been electrically charged to improve the filtration performance (see, for example, U.S. Pat. No. 6,524,488 to Insley et al.).
During use, electret filters frequently become loaded with particles and contaminants that interfere with the filtering capabilities of the electret filter. Liquid aerosols, for example, particularly oily aerosols, may cause electret filters to lose their electret-enhanced filtering efficiency (see, U.S. Pat. No. 6,627,563 to Huberty).
Numerous methods have been developed to counter this filtering efficiency loss. One method includes adding additional layers of nonwoven polymeric web to the filter. This approach, however, can increase the pressure drop across the electret filter and can add to its weight and bulk. When the electret filter is used in a personal respiratory protection device, these drawbacks can be particularly troublesome. Increased pressure drop, for example, results in increased breathing resistance, making the respirator more uncomfortable to wear. Another method for improving resistance to oily-mist aerosols, includes adding a melt processable fluorochemical additive such as a fluorochemical oxazolidinone, a fluorochemical piperazine, or a perfluorinated alkane to the polymer during the creation of the polymeric fibrous article—see, for example, U.S. Pat. Nos. 5,025,052 and 5,099,026 to Crater et al. and U.S. Pat. Nos. 5,411,576 and 5,472,481 to Jones et al. The fluorochemicals are melt processable, that is they suffer substantially no degradation under the melt processing conditions that are used to form the fibers in the electret web—see also U.S. Pat. No. 5,908,598 to Rousseau et al. In addition to a melt-processing method, fluorinated electrets also have been made by placing a polymeric article in an atmosphere that contains a fluorine-containing species and an inert gas and then applying an electrical discharge to modify the surface chemistry of the polymeric article. The electrical discharge may be in the form of a plasma such as an AC corona discharge. The plasma fluorination process causes fluorine atoms to become present on the surface of the polymeric article. The fluorinated polymeric article may be electrically charged using, for example, the hydrocharging techniques mentioned above. The plasma fluorination process is described in a number of U.S. Patents to Jones/Lyons et al.: U.S. Pat. Nos. 6,397,458, 6,398,847, 6,409,806, 6,432,175, 6,562,112, 6,660,210, and 6,808,551. Other publications that disclose fluorination techniques include: U.S. Pat. Nos. 6,419,871, 6,238,466, 6,214,094, 6,213,122, 5,908,598, 4,557,945, 4,508,781, and 4,264,750; U.S. Publications US 2003/0134515 A1 and US 2002/0174869 A1; and International Publication WO 01/07144.
Although fluorinated electrets are suitable for many filtration applications, some filters require enhanced thermal stability to meet product specifications, for example, military specifications and NIOSH requirements—see NIOSH, Statement of Standard for Chemical, Biological, Radiological, and Nuclear (CBRN) Air-Purifying Escape Respirator, Attachment A, Sep. 30, 2003 and NIOSH, Statement of Standard for Chemical, Biological, Radiological, and Nuclear (CBRN) Full Facepiece Air Purifying Respirator (APR), Appendix A, Apr. 4, 2003. Applications exist where the electret filter media should be resistant to charge degradations at high temperatures for prolonged periods. The present invention described below addresses this need and accordingly provides an electret article that has good oily mist filtration properties despite being exposed to high temperatures for extended time periods.